Physicists Report Neutrinos Have Mass

Photonics SpectraJul 1998
In a discovery described as monumental, scientists report that neutrinos have a mass -- an observation that could alter current theories on dark matter and provide clues to the origin of the universe.
The findings came from a $100 million detector called Super Kamiokande, buried in an old zinc mine 3250 ft under Mount Ikena in Japan. The detector contains 12.5 million gallons of ultrapure water, and underneath the water is an acre of photomultiplier tubes manufactured by Hamamatsu Corp. The photomultipliers record those occasions when the elusive neutrinos strike an atom of water, producing either a muon or an electron. Scientists record an average of only 5.5 neutrino strikes each day.
Researchers observed a difference between the number of neutrinos at the Super Kamiokande site and those observed at another detector on the opposite side of Earth. To explain the discrepancy, scientists theorized that neutrinos can oscillate from one type of neutrino to another. This ability demonstrates that the particles have a mass.